Your plumber is right. Just because the drain is connected to the main stack does not mean its vented properly. Every drain needs a vent that goes up to the roof to relieve the pressure created by putting water into the sink. Since the water must first displace the air in the pipe. Typically it pushes it ahed of it and sends it up the vent pipe.
If you live in a coop, there may be other apartments above you. When any fixture flushes from the apartment above you the pressure created on the stack has to be relieved, if your sink has its own relief vent, it sends the pressure from the upstairs fixtures to the roof. If there is no vent, it bubbles sewer gas out the trap.
A trap in a sink has 2 inch water seal which is about .03 pounds of pressure. When 1.5 gallons of water flushes down the stack it weight 8.33 pounds per gallon and as it travels, it increases its pressure and speed as it falls. Think of a paratrooper falling from a plane, he picks up speed till he reaches about 220 mph. Water reaches the same speed in a pipe falling straight down. Since you have only .03 pounds in your trap, the increased pressure from the falling water overcomes the pressure of the trap and pushes out the stinky gasses brewing in the sewer pipe.
Now however saying all that, if you are the uppermost apartment and you are within 42 inches of the stack, you do not need a vent as long as the drain remains horizontal from the trap to the stack.
I hope that answers your questions.